To present your Overflow diagram you have to click on the Present icon at the bottom right of the Editor. From there you can exit back to the Editor by pressing Esc.
Your presentation is auto-generated from your Overflow diagram. It is interactive in the sense that screens, images, and shapes become focal points and any element (or screen layer) that has an outgoing connector becomes active and can lead you to the destination element of that connector (upon clicking).
You can click to navigate to screens, images & shapes or on connectors to follow it up to its destination. Using the arrow keys will transition over to nearby elements in the specified direction.
A beautiful zoom-out - zoom-in transition while navigating makes this a captivating experience, while at the same time giving you and your audience a glimpse of the surrounding elements. You can turn this off if you like with Cmd + f.
You can hide all connectors from your view by pressing the Tab key.
You can highlight interactive areas by holding the Shift key.
To get a bird's-eye view of your Overflow diagram, you can press Cmd + 1.
You can navigate through your boards using the menu on the left (shown in the image below), or Cmd + ↑ and Cmd + ↓
Note: If you want to exclude an element from being active in the presentation (i.e. not be a focal point), you can choose the relevant option ("Set as Inactive") from the context menu in the Editor while having the particular element selected.